1. Field of the Invention
The present invention relates to optical devices, such as wavelength routers and optical multiplexers, used in light-based telecommunications and computer networks.
2. Description of the Related Art
A wavelength router is a type of optical device that selectively routes light of a particular wavelength from an input port to an output port. When used to route light of differing wavelengths from more than one input port and/or to more than one output port, a wavelength router can be used to operate as an optical multiplexer and/or demultiplexer.
FIG. 1 shows a schematic diagram of a typical wavelength router 100 implemented as an integrated device formed on a suitable substrate 102 (e.g., silicon or silica). Router 100 has a plurality of input waveguides 106 adapted to receive light from one or more incoming optical fibers that may be connected to one or more of the input ports 104. Router 100 also has a plurality of output waveguides 114 adapted to transmit light to one or more outgoing optical fibers that may be connected to one or more of the output ports 116. Between the input and output waveguides are two free spaces 108 and 112 separated by a set of waveguides that form the arms 110 of the router.
In operation, light received at one of the input ports 104 is transmitted along the corresponding input waveguide 106 to free space 108. Light entering free space 108 gets radiated for receipt by--and transmission along--each of the router arms 110 towards free space 112. Light entering free space 112 gets radiated towards the output waveguides 114.
Wavelength router 100 is preferably designed such that all of the optical distances from a particular location at the input side of free space 108 (i.e., where one particular of the input waveguides 106 meets free space 108) along each router arm 110 to a particular location on the output side of free space 112 (i.e., where one particular of the output waveguides 114 meets free space 112) differ by an integer multiple of a particular wavelength. As such, light of that particular wavelength entering free space 108 from that particular input waveguide 106 will be focused on the output side of free space 112 at that particular output waveguide 114. That is, light of that particular wavelength will constructively interfere (i.e., add in phase) at that particular output waveguide location, and substantially destructively interfere at all other output waveguide locations. Moreover, light of most other wavelengths will not, in general, be focused (i.e., will effectively destructively interfere) at that particular output waveguide location. As such, wavelength router 100 can be used as an optical passband filter.
Furthermore, to the extent that wavelength router 100 can be designed to focus light having different wavelengths at different output waveguide locations on the output side of free space 112, router 100 can operate as a one-to-many optical multiplexer that can receive light of different wavelengths from a single incoming optical fiber and selectively transmit those different frequencies to different output ports for propagation along different outgoing optical fibers. Similarly, router 100 can be further designed to operate as a many-to-one optical demultiplexer or many-to-many optical multiplexer that receives different wavelength light from different incoming optical fibers for transmission to different outgoing optical fibers. Moreover, router 100 may be a symmetric optical device that can be operated in either direction (i.e., either from left to right or from right to left in FIG. 1). Typically, the router is realized using silica waveguides deposited on a thick substrate of quartz or silicon.
As shown in FIG. 1, router 100 is an integrated device formed on a substrate 102 that is rectangular in shape. The rectangular substrate shape results from standard integrated circuit (IC) manufacturing techniques in which one or more devices are fabricated on a circular wafer and then separated by cutting the wafer along straight dicing lines using a high-speed circular saw.
FIG. 2 shows a schematic diagram of three routers similar to router 100 of FIG. 1 as they would be formed during conventional manufacturing on a single wafer. To separate the three routers, the wafer is cut along the straight dicing lines indicated in the drawing using a high-speed circular saw.
One of the advantages of using a high-speed circular saw to make straight dicing cuts is that the resulting rectangular-shaped routers are relatively free from cracks and fissures that will propagate over time into the waveguides and free spaces of the router and thereby adversely affect the ability of the router to operate properly. As a result, such rectangular devices have relatively long operational lifetimes.
There are drawbacks, however, to the rectangular devices that result from using high-speed circular saws to make straight dicing cuts. One drawback is the relatively low manufacturing yield (i.e., the number of devices per wafer). Devices such as wavelength router 100 of FIG. 1 are typically large (e.g., 2.times.7 cm). As a result, a typical wafer will yield only two or three rectangular devices.
Furthermore, rectangular devices such as wavelength router 100 of FIG. 1 can produce undesirable crosstalk from stray light energy that passes from an incoming optical fiber directly through the substrate to an outgoing optical fiber due, for example, to misalignment of the incoming optical fiber at the input port, as shown schematically in FIG. 3. This stray light can lead to crosstalk in which light is inappropriately routed from one optical fiber to another. Such stray light may result from imperfect mounting of the optical fibers to the device.
What is needed is a scheme for avoiding the limitations of the prior art (e.g., low yield and crosstalk) without jeopardizing the advantages of the prior art (e.g., operational lifetime).